FIG. 1 very schematically shows the electric connection of a system 1 of supply of a load 2 (Q) from an A.C. voltage Vin, provided with a bridge 5 for rectifying voltage Vin and with a circuit 4 of supply of load 2 from an approximately D.C. voltage Vout. Voltage Vout is taken across a capacitor C receiving a rectified A.C. output voltage of bridge 5.
Input voltage Vin of system 1 is an A.C. voltage coming, for example, from a variator 3 of an A.C. supply voltage Vac, for example, the mains voltage.
When it is desired to vary the supply power of a load having a resistive input impedance, a phase angle or phase angle switching variator is generally used to modulate the power transmitted to load 2.
Although such a phase angle variator is well adapted to applications for which load 2 is of resistive type and does not require a supply from a recovery of a D.C. voltage, conversely to what is shown in FIG. 1, such a phase angle variator raises several problems in the case of a capacitive input impedance load.
A first problem is that, for the phase angle variation to translate as a power variation of load 2, the approximately D.C. voltage Vout has to follow the power variations linked to the phase angle variation. As a result, circuit 4 used to supply load 2 sees its own supply vary, which can cause malfunctions due to the supply needs of the components of circuit 4. For example, if circuit 4 forms a switched-mode converter used to supply a load 2 formed of a fluorescent lamp, a variation of voltage Vout adversely affects the proper operation of the switched-mode converter.
Further, a switching in the charge area of a capacitor constitutive of the input impedance results in a significant effective current, which is not desirable.
Accordingly, for loads having a capacitive input impedance, other means than the phase angle variation are conventionally used to act upon the operation of system 1.
In a conventional circuit 4 such as shown in FIG. 1, the power variation function is generally performed from an analog low voltage input E of circuit 4. The signal applied to terminal E is used, for example in an application to a fluorescent lamp, to modify the frequency of the alternating current provided by the switched-mode converter to vary the light intensity. Terminal E of light intensity dimming control is meant to be controlled by an external variator 3 setting a control voltage generally included between 0 and 5 volts and proportional to the desired light intensity.
A major disadvantage of this variation solution is the need for a low voltage link between system 1 of control of load 2 (here, a fluorescent lamp) and a generally remote mechanical potentiometer-switch (variator 3). As illustrated in FIG. 1, in addition to the two conductors 8, 9 (phase and neutral) of A.C. supply Vin, two low voltage conductors (dotted lines 7) indeed have to be provided between a switch 3 including a dimmer and electronic system 1 of control of load 2.
Another conventional solution to transmit a light intensity order to a load supply control circuit 4 consists of performing a modulation of the carrier current, that is, modulating the alternating supply current with a high frequency signal transmitting the light intensity order. Such a solution requires, on the side of dimmer 3, a carrier current modulation system (not shown) to transmit the order and, on the side of system 1, a demodulator (not shown) for extracting the power order from the A.C. supply.
Such a solution has the advantage of avoiding the need for an additional link 7. However, it has the disadvantage of being particularly complex and expensive to implement.